It is known that brittle materials can be divided by introducing thermally induced stresses by means of a laser beam. To this end, a laser beam is directed onto the surface of the material, along the desired scored line (working line), so as to produce compressive stresses, and subsequently, a coolant jet is applied to the heated zone, which produces tensile stresses.
If the resultant stress difference is greater than the ultimate stress of the material and there is a defect in the working line, a tear in the material is initiated, starting from the defect.
If such a defect is not available because there are no existing microcracks, e.g., originating from a mechanical cutting edge, an initial defect must be targetedly produced, e.g., by means of a diamond cutting wheel or an accurately focusable laser with high energy density. As to the latter, the working laser or a second laser can be used.
Depending on the internal structure of the material, it is possible to make a targeted deep cut or a break-off cut that completely penetrates the material.
To divide a plane-parallel plate into a plurality of identical individual rectangular plates, e.g., in the production of display panels for mobile telephones or the like or in the production of chips, the plane-parallel plate is initially divided along a plurality of first working lines that run parallel to one another and subsequently, perpendicular thereto at an angle of intersection of 90°, the plane-parallel plate is divided along a plurality of second cutting lines that run parallel to one another. Determined by the points of intersections of the working lines that intersect at an angle of 90°, this leads to individual rectangular plates, the side lengths of which are determined by the distances between the working lines.
The method is not limited to dividing plane-parallel plates into individual rectangular plates but to dividing them in a lattice-like pattern, with the rectangular shape being useful for the applications mentioned above.
The German Patent Specification DE 100 41 519 C1 acknowledges the problem that this type of method entails, which is that it is difficult to produce the initial defects at the points of intersection. According to this document, dividing a plane-parallel plate, beginning at an edge of the plane-parallel plate along a first working direction, into a plurality of plate strips is not a problem since, in order to produce the initial defect required for each scored line, it is possible to apply a cutting wheel with high accuracy to the edge of the plane-parallel plate.
In contrast, to create the initial defects along the resultant edges of the plate strips—or more accurately, at the points of intersections—the cutting wheel must be applied from above in order to divide the adjoining plate strips into individual plates. According to this document, due to the method used, this would result in damage to the converging edges of the individual plates.
The German Patent Specification DE 100 41 519 C1 proposes that the plate strips cut from a plane-parallel glass plate be moved by a specified distance apart from one another before dividing them into individual plates. This makes it possible for the cutting wheel to be accurately applied to the edges of the plate strips as well.
To this end, the plane-parallel glass plate is mounted on a cutting table which comprises several plate segments that can be moved apart from one another. The cutting table preferably has a dedicated vacuum system for clamping the plane-parallel glass plate and the cut parts. Special advantages are promised if each table segment has a dedicated, separately controllable vacuum field. This, it is said, makes it possible to mount and detach the cut plate strips separately, thereby avoiding a negative influence of the glass, which may potentially have become warped because of the vacuum, on the subsequent laser cutting procedure.
Apart from the high degree of mechanical complexity, in combination with a cutting table of the type described in this document, a device with such a cutting table has the crucial disadvantage that the width of the plate strips cannot be freely chosen, but that this width is instead defined by the width of the separate table segments. Thus, the choosable width of the plate strips is predetermined by the width of a table segment or a multiple of the width of a table segment.
In addition, the thickness of the plane-parallel glass plates is limited to the minimum thickness of the plane-parallel glass plates that are generally directly mounted on the cutting table.
Plane-parallel plates in a thickness range from 100-300 μm, such as are produced in the form of thin wafers from silicon or gallium arsenide, are generally only bonded to stretch films. After completion of processing and separating the chips, the stretch film is stretched in order to avoid mechanical damage to the chip edges during transport, on the one hand, and to be able to detach the chips one by one from the film, without allowing them to come into contact with the adjacent chips, on the other hand.
According to current practice, the chips are primarily separated by mechanically sawing them apart. The German Patent Application DE 10 2005 012 144 A1 describes a device, by means of which scored lines, which are arranged in a lattice-like pattern, are applied to a wafer that is mounted on a film, using a laser beam which ablates material. However, the ablation of material invariably entails the risk that the surface will be contaminated.
The applicant of the present invention carried out tests in which an attempt was made to divide a wafer mounted on a film in a lattice-like pattern by introducing thermally induced stresses by means of a laser beam, as already briefly described in the introduction.
This led to quality defects at the points of intersection between the working lines along the first working direction and the working lines along the second working direction. However, these defects are not attributable to the fact that the tool, e.g., a laser or a diamond cutting wheel, was applied from above to the surface of the plane-parallel plate at the point of intersection, such as described in the prior-art German Patent Specification DE 100 41 519 C1, but are instead attributable to the fact that the resultant stresses in the adjacent strip plates reciprocally influence one another.
In the area of transition to the next plate strips, the hot spot that precedes the coolant spot produced causes the material to expand, which causes the hot spot to push against the previous plate strips, thereby causing the scored line on the end of the plate strips to be deflected from its intended direction of separation. Accordingly, given a specified point of intersection of two scored lines that intersect at an angle of 90°, no precise right angle forms along the edges of the individual plates.
The Japanese Patent JP 10-268275 A describes a method of cutting glass substrates and a device suitable for this purpose. The glass substrate to be cut is mounted on a holding fixture by attaching it by suction over a porous stretchable body. By stretching the body, the cutting properties are improved since during the cutting operation, tensile stresses are generated on the surface of the glass substrate.
This method is not suitable for spatially separating cut substrate segments.